N-(phosphonomethyl)glycine (glyphosate) and its salts are conveniently applied as a component in aqueous, post-emergent herbicide formulations. As such, they are particularly useful as highly effective and commercially important broad-spectrum herbicides for killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation and aquatic plants.
Various methods for preparation of glyphosate have been developed. One method includes the catalyzed liquid phase oxidative cleavage of a carboxymethyl substituent from an N-(phosphonomethyl)iminodiacetic acid (PMIDA) substrate. Over the years, a wide variety of methods and reactor systems have been disclosed for conducting this oxidation reaction. See generally, Franz, et al., Glyphosate: A Unique Global Herbicide (ACS Monograph 189, 1997) at pp. 233-62 (and references cited therein); Franz, U.S. Pat. No. 3,950,402; Hershman, U.S. Pat. No. 3,969,398; Felthouse, U.S. Pat. No. 4,582,650; Chou, U.S. Pat. No. 4,624,937; Chou, U.S. Pat. No. 4,696,772; Ramon et al., U.S. Pat. No. 5,179,228; Siebenhaar et al., International Publication No. WO 00/01707; Ebner et al., U.S. Pat. No. 6,417,133; Leiber et al., U.S. Pat. No. 6,586,621; and Haupfear et al., U.S. Pat. No. 7,015,351.
The reaction may be conducted in either a batch or continuous oxidation reactor system in the presence of a catalyst that typically comprises particulate carbon, or a noble metal such as platinum on a particulate carbon support. The catalyst is usually slurried in an aqueous solution of PMIDA within a stirred tank reactor, and molecular oxygen is introduced into the reactor to serve as the oxidizing agent. The reaction is exothermic. The liquid phase oxidation of a PMIDA substrate typically produces a reaction mixture containing water and various impurities besides the desired N-(phosphonomethyl)glycine product. These impurities may include, for example, various by-products, unreacted starting materials, as well as impurities present in the starting materials. Representative examples of impurities present in N-(phosphonomethyl)glycine product reaction mixtures include, for example, unreacted PMIDA substrate, N-formyl-N-(phosphonomethyl)glycine (NFG), phosphoric acid, phosphorous acid, N-methyl-N-(phosphonomethyl)glycine (NMG), glycine, aminomethylphosphonic acid (AMPA), methyl aminomethylphosphonic acid (MAMPA), iminodiacetic acid (IDA), imino-bis-(methylene)-bis-phosphonic acid (iminobis), formaldehyde, formic acid, chlorides, and ammonium sulfate.
Other methods for preparation of glyphosate utilize different starting materials including, for example, glycine, which is used in the so-called “alkylphosphite process.” See, for example, Chinese Patent Disclosure No. CN 1629112A. Such methods are often referred to elsewhere and herein as the “glycine method” or “glycine route.” These methods generally comprise dissolving formaldehyde or paraformaldehyde in a solvent (typically methanol (MeOH)) containing triethylamine followed by addition of glycine. After addition and dissolution of glycine, dimethylphosphite is added, followed by addition of hydrochloric acid (HCl) to produce a mixture of HCl, glyphosate, and methyl chloride. Neutralization by addition of a base provides the glyphosate salt(s). Alternatively, trimethylphosphite may be used as the starting material. Methyl chloride may be recovered and utilized in the manufacture of organosilane products as described, for example, in Chinese Patent Disclosure No. CN 1446782. As with the PMIDA-based method described above, preparation of glyphosate in this manner results in a reaction mixture containing a variety of impurities including, for example, glycine, N,N-bis(phosphonomethyl)glycine (glyphosine), phosphorous acid, phosphoric acid, hydroxymethylphosphonic acid, and triethylamine hydrochloride (Et3N.HCl). Recovery of triethylamine (e.g., using a strong base such as sodium hydroxide (NaOH)) can improve the process economics.
Glyphosate may be produced from glycine, e.g., by a process as described in U.S. Pat. No. 4,486,359, which is expressly incorporated in its entirety herein by reference for all relevant purposes. In this process, glycine is initially reacted with paraformaldehyde in the presence of triethylamine to produce N,N-bis(hydroxymethyl)glycine. The reaction is conducted in a methanol medium, typically at MeOH reflux temperature (i.e., about 65° C.). The N,N-bis(hydroxymethyl)glycine intermediate is reacted with dimethyl phosphite to yield an ester, which U.S. Pat. No. 4,486,359 characterizes as the methyl ester of glyphosate. The ester is hydrolyzed in HCl to glyphosate acid. This product generally has a glyphosine content in excess of 0.010 wt. %, more typically between about 0.05% and about 2% on a glyphosate, a.e., basis. Commercial sources of glycine process glyphosate may commonly contain between about 0.2% and about 1.5% by weight glyphosine and between about 0.05% and about 0.5% by weight glycine, more typically between about 0.3 and about 1% by weight glyphosine and between about 0.1 and about 0.3% by weight glycine, all on a glyphosate, a.e., basis.
In an alternative to the process of U.S. Pat. No. 4,486,359, Japanese Published Application Hei 9-227583 (application no. Hei-9-6881) describes a process in which the reaction between paraformaldehyde and glycine may be conducted in the presence of tributylamine rather than triethylamine, and the ester intermediate may be hydrolyzed in an alkaline medium such as NaOH rather than in acidic medium such as HCl. The Japanese patent publication reports that the base hydrolysis may produce a product of lower glyphosine content than the product of the process of U.S. Pat. No. 4,486,359.
In conducting the process of Japanese Published Application Hei 9-227583, a source of formaldehyde, preferably paraformaldehyde is mixed with a reaction medium comprising C1 to C4 alcohol at moderately elevated temperature, tributylamine is added to the resulting solution and the mixture preferably agitated at about 35° C. to 50° C. for typically 30 to 60 minutes. Glycine is added to the alcohol medium in a proportion which preferably assures a formaldehyde to glycine molar ratio from about 1 to 5, and the glycine is preferably completely dissolved in the medium. Preferably, the molar ratio of tributylamine to glycine is from about 0.5 to about 3. The temperature is maintained at least about 30° C., preferably between about 50° C. and about 60° C. for typically about 10 to 60 minutes, resulting in reaction of glycine with formaldehyde to form the tributylamine salt of N-methylolglycine. A dialkylphosphite, e.g., dimethylphosphite, is then added to the solution under agitation at elevated temperature, preferably at least about 50° C., more typically about 65° C. to about 80° C., conveniently under alcohol reflux, preferably at a molar ratio to N-methylolglycine from about 0.6 to about 2.0. The dialkylphosphite condenses with the tributylamine salt of N-methylolglycine to yield an ester intermediate depicted in the Japanese patent publication as the dialkyl ester of the tributylamine carboxylate salt of glyphosate. Addition of a strong base such as NaOH to this solution saponifies the ester, liberates tributylamine and forms the sodium salt of glyphosate. The reaction mixture separates into two liquid phases, yielding an upper layer containing tributylamine and a lower layer comprising a solution of sodium salt of glyphosate. Tributylamine may be recovered from the upper layer for recycle. The lower layer may be acidified to crystallize glyphosate acid.
The alkaline hydrolysis may be conducted with a strong base comprising a desired countercation such as, e.g., potassium hydroxide (KOH), as a step in the preparation of an aqueous concentrate of the potassium salt of glyphosate. Where the phase separation is carried out under conditions that assure substantially quantitative partition of tributylamine to the upper layer, the lower layer may be used directly in the preparation of an aqueous glyphosate concentrate comprising the potassium salt. Alternatively, the glyphosate salt may be acidified to precipitate glyphosate acid, and the glyphosate acid separated by filtration or centrifugation and washed, and the washed glyphosate wet cake reslurried with water and base to produce the desired salt. In the latter process, the advantage of using KOH for the conversion of intermediate ester to glyphosate salt is diminished. Where triethylamine is used as the alkylamine, it can be quantitatively removed by distillation of the hydrolyzate, which may in certain instances facilitate direct preparation of a concentrate of the glyphosate salt of the base used for the conversion of the intermediate ester. Preferably, the concentrate comprises at least about 360 grams per liter (g/L) glyphosate on an acid equivalent (a.e.) basis.
Regardless of the precise method by which a glyphosate product is manufactured, a concentrated glyphosate product, or wet cake can be prepared from the resulting reaction product solution. Preparation of the glyphosate wet cake also produces a filtrate, or mother liquor that contains various impurities, along with a portion of the glyphosate product not isolated in the wet cake. The glyphosate present in the filtrate, or mother liquor may represent up to 10% (e.g., from 5% to 10%) of the glyphosate produced.
In addition to the above-noted processes (e.g. PMIDA-based and glycine routes), glyphosate product may be manufactured by processes that use AMPA as the substrate. Both glycine and AMPA-based processes generate a profile of by-products and impurities that is somewhat different from that of the PMIDA oxidation process. For example, the product of the glycine process most typically contains glyphosine in a concentration greater than about 0.010 wt. %, more typically at least about 0.1 wt. %, and most typically in the range of about 0.3 to about 1 wt. %, all on a glyphosate a.e. basis. The product of the AMPA-based process may have a modest to significant fraction of unreacted AMPA, though the product of the PMIDA process can have a comparable AMPA content. The glycine content of the AMPA process product is generally significantly lower than 0.02 wt. % on a glyphosate, a.e., basis.
To capitalize on the glyphosate present in the mother liquor, glyphosate products have been prepared by adding relatively pure glyphosate to the mother liquor to produce a solution of glyphosate containing, for example, approximately 10 wt. % of glyphosate. Unfortunately, however, glyphosate product solutions prepared in this manner typically contain a comparable level of salt (e.g., sodium chloride) along with other impurities associated with the manufacture of glyphosate. Utilizing a glyphosate product containing such high levels of sodium chloride in agricultural applications is undesired for environmental reasons.
Recovery of valuable products from the filtrate, or mother liquor, produced during glyphosate manufacture would improve the overall economics of glyphosate manufacturing processes while avoiding the undesired environmental consequences associated with current practices. More particularly, providing effective recovery of phosphorus and salt values from the filtrate would substantially reduce, and preferably eliminate completely the desirability of preparing glyphosate products of high impurity (e.g., sodium chloride) content directly from the mother liquor.
In addition, commercial processes for manufacture of glyphosate may include deepwell injection of various waste streams, including the mother liquor resulting from wet cake production. Deepwell injection has been and may be practiced in an environmentally responsible manner. However, a method for treatment of waste streams that provides recovery of valuable products as an alternative to deepwell injection would be desirable in the event that deepwell injection of waste streams from glyphosate manufacture is not permitted or commercially practical.
Accordingly, there exists an unfulfilled need in the art for processes for recovery of values from the mother liquor generated in production of glyphosate wet cake and other aqueous waste streams generated in the manufacture of glyphosate, as well as from aqueous process streams generated in the manufacture of glyphosate precursors (e.g. PMIDA).